Optical recording medium

ABSTRACT

On one surface of a disk substrate, a first dielectric layer, a thin recording layer, a second dielectric layer and a reflecting layer are cumulatively formed. The second dielectric layer is thinner than the first dielectric layer and is 35-70 nm thick. The reflecting layer is 70-120 nm thick. The total thickness of the four layers is 250-430 nm. The compressire stress of the layers is offset by tensile stress generated by an overcoat resin protective layer, formed on the reflecting layer. A method of using the optical recording medium applies the MCAV (Modified Constant Angular Velocity) recording method which records information by changing recording frequencies in accordance with linear velocity. The linear velocity is 5-12 m/s. A ratio of an inner circumference recording pulse width to an outer circumference recording pulse width is 1.2 or more.

FIELD OF THE INVENTION

The invention relates to an optical recording medium (indicated as adisk hereinafter) which can record, reproduce and delete information athigh density and capacity by a laser beam or the like, and furtherrelates to a method of using the optical recording medium.

BACKGROUND OF THE INVENTION

Reloadable disks are conventionally used for many purposes. A disktypically includes a disk-type transparent substrate, and a thinrecording layer on one surface of the substrate. Data is repeatedlyrecorded and deleted by reversibly changing the optical density of thethin recording layer. The thin recording layer of the disk iscrystallized in advance, and is then heated, melted and quenched byirradiating the thin recording layer with about a 1 μm laser beam andchanging the intensity of the beam. As a result, the layer becomesamorphous and records information. In addition, the amorphous recordingthin layer is crystallized by raising the temperature of the layer in arange between the crystallization temperature and the melting point andthen annealing the layer, thus deleting information.

When a resin substrate, for example, is used for the disk-thin recordinglayer is formed directly on the substrate, the substrate is heated to ahigh temperature in a minute section of about 1 μm by recording anddeleting and is thus deformed. Therefore, in general, a dielectric layeris formed as a heat insulating layer between the substrate and the thinrecording layer and between the thin recording layer and a layerprotecting the thin recording layer (protective layer), thus preventingthermal deformation of the substrate. Since the temperature rise,quenching and annealing properties of the thin recording layer vary dueto the heat conduction properties of the protective layer, recording anddeleting characteristics of the disk are improved by selectingpreferable materials and layer composition. Furthermore, the recordingand deleting characteristics of the disk are improved by forming areflecting layer on the surface of the dielectric layer facing theprotective layer and utilizing the multiple interference of a laserbeam. This four-layer disk structure is generally well known.

A rapid cooling disk structure is proposed for the four-layer reloadabledisk. In the rapid cooling disk structure, the dielectric layer betweenthe thin recording layer and the reflecting layer (identified as asecond dielectric layer hereinafter) is thinned so that heat generatedin the thin recording layer during recording and deleting is quicklyreleased to the reflecting layer. The rapid cooling disk structure hasan advantage in that the outer limits of the erase rate and powerimprove since the temperature of the dielectric layers on both sides ofthe thin recording layer rises by widely dispersing the heat from thethin recording layer. It is also advantageous in making the thinrecording layer amorphous since the layer is cooled quickly in thisstructure. Japanese Patent Application No. Sho 63-207040 (PublishedUnexamined (Laid-open) Japanese Patent Application No. Hei 02-056746)discloses a rapid cooling disk structure. In this application, thesecond dielectric layer is thinner than the dielectric layer between thedisk substrate and the thin recording layer (identified as a firstdielectric layer hereinafter) and is 30 nm thick or less.

However, when the thickness of the second dielectric layer is 30 nm orless, the recording and deleting sensitivities of the disk decline, sothat an expensive high power semiconductor laser has to be used. Inaddition, the first and the second dielectric layers are under thermalstress, expanding and shrinking due to rapid heating at 400° C. orhigher and cooling for repeated recording and deleting. Although thesecond dielectric layer has a smaller heat load than the firstdielectric layer, the layer repeatedly receives thermal stress. Thelayer should thus have a proper thickness. The thickness of the seconddielectric layer has to be selected in consideration not only of theheat conduction but also of optical properties.

SUMMARY OF THE INVENTION

It is an object of this invention to solve the above-mentioned problemsby providing an optical recording medium, which has high recordingsensitivity and is excellent in recording and deleting repeatedly, and amethod of using the optical recording medium.

In order to accomplish these and other objects and advantages, theoptical recording medium includes a transparent substrate, a firstdielectric layer formed on one surface of the transparent substrate, athin recording layer which is formed on the first dielectric layer, asecond dielectric film formed on the thin recording layer, and areflecting layer formed on the second dielectric layer. The thinrecording layer has properties of becoming amorphous after itstemperature is increased, then melting and quenching by absorbing energyfrom the irradiation of a laser beam, and of crystallizing its amorphousstate by temperature rise. The second dielectric layer is thinner thanthe first dielectric layer and is 35-70 nm thick, and the thickness ofthe reflecting layer is 70-120 nm.

It is preferable that the thin recording layer is 18-30 nm thick. Thefirst dielectric layer preferably is 140-210 nm thick. The first and thesecond dielectric layers preferably contain ZnS at 60-95 mol % and SiO₂at 5-40 mol %.

The reflecting layer preferably contains Al as a main material and atleast one metal selected from the group comprising Ti, Ni, Cr, Cu, Ag,Au, Pt, Mg, Si and Mo.

The thin recording layer preferably contains Te, Ge and Sb, andpreferably also contains nitrogen. The thin recording layer preferablycontains GeTe, Sb₂ Te₃, Sb and nitrogen.

The thin recording layer is preferably composed so that 0.2≦b≦0.5 whereb is the mol ratio of Sb/Sb₂ Te₃.

The optical recording medium preferably further includes a 3-15 μm thickovercoat protective layer on the reflecting layer, and that theprotective layer, which preferably generates tensile stress.

It is preferable that the overcoat resin is sealed for stressrelaxation.

The first dielectric layer, the thin recording layer, the seconddielectric layer and the reflecting layer are preferably 250-430 nmthick altogether, and preferably have 0.5×10⁹ dyn/cm² or lesscompressire stress.

The optical recording medium preferably is recorded with information onone surface, and an angle where a tangent of the surface and thehorizontal reference plane of the medium intersect each other ispreferably 1-2 mrad so as to protrude the surface.

The transparent substrate preferably has a convex surface. An anglewhere a tangent of the convex surface and the horizontal reference planeof the substrate intersect each other is preferably 1-2 mrad, and thefirst dielectric layer, the thin recording layer, the second dielectriclayer and the reflecting layer are preferably cumulatively formed on theconvex surface.

The method of using an optical recording medium of the invention appliesthe MCAV (Modified Constant Angular Velocity) recording method whichrecords by varying recording frequencies in accordance with linearvelocity. The linear velocity is 5-12 m/s. The recording pulse width ofthe inner circumference is larger than that of the outer circumference,and the ratio of the inner circumference recording pulse width to theouter circumference recording pulse width is 1.2 or above. The opticalrecording medium includes a transparent substrate, a first dielectriclayer formed on one surface of the transparent substrate, a thinrecording layer formed on the first dielectric layer, a seconddielectric film formed on the thin recording layer, and a reflectinglayer formed on the second dielectric layer. The thin recording layerhas properties of becoming amorphous after its temperature is increased,then melting and quenching by absorbing energy from the irradiation of alaser beam, and of crystallizing its amorphous state by temperaturerise. The second dielectric layer is thinner than the first dielectriclayer and is 35-70 nm thick, and the thickness of the reflecting layeris 70-120 nm.

The recording pulse width preferably is 40-60 ns at 5 m/s linearvelocity and is 30-40 ns at 12 m/s.

The MCAV recording method preferably is applied so as to recordinformation by varying a record starting point at the maximum of 7.75 μmin 5-12 m/s linear velocity.

The optical recording medium comprises the transparent substrate, thefirst dielectric layer, the thin recording layer, the second dielectricfilm, and the reflecting layer. The thin recording layer has theabove-mentioned properties. The second dielectric layer is thinner thanthe first dielectric layer and is 35-70 nm thick, and the thickness ofthe reflecting layer is 70-120 nm. Thus, the optical recording mediumhas high recording sensitivity and excellent repeated recording anddeleting properties.

A mixed material of ZnS and SiO₂ having excellent heat resistingproperties and relatively small heat conduction is used for the firstand the second dielectric layers. The second dielectric layer is 35-70nm thick, and has not only excellent optical properties but alsomechanical strength. The second dielectric layer widens the distancebetween the thin recording layer and the reflecting layer, and slowsdown the cooling speed of the thin recording layer. As a result, therecording sensitivity of the optical recording medium is improved.

The thickness and heat capacity of the reflecting layer is reduced whilethe mechanical strength and optical properties of the layer aremaintained, so that recording sensitivity improves. In other words, thethermal properties and recording sensitivity of the optical recordingmedium can be controlled by choosing a preferable layer thickness. Sincethe reflecting layer utilizes the multiple interference of a laser beam,the layer can prevent deterioration and crystal growth in a hightemperature and humidity environment by containing at least one elementselected from the group comprising Ti, Ni, Cr, Cu, Ag, Au, Pt, Mg, Siand Mo in addition to Al.

The thin recording layer contains Te, Ge and Sb; To, Go, Sb andnitrogen; GeTe, Sb₂ Te₃ and Sb; or GeTe, Sb₂ Te₃, Sb and nitrogen. Thethin recording layer is composed so that 0.2≦b≦0.5 where b is the molratio of Sb/Sb₂ Te₃. It is not preferable to have b greater than 0.5since the erase rate declines due to the decrease in crystallizationspeed. When b is less than 0.2, there is a problem in that the recordingamplitude of an inner circumference having a low linear velocity isreduced. By adding nitrogen to the thin recording layer, the heatconductivity of the layer is reduced. The crystallization speed of thelayer is also reduced by increasing the amount of Sb. As a result, therecording sensitivity of the optical recording medium is improved.

In the MCAV recording method, the recording pulse width of the innercircumference is set larger than that of the outer circumference at 5-12m/s linear velocity, so that heat up temperature at the inner and theouter circumferences during recording is set equal. Thus, marks ofalmost the same size are formed, thereby reducing mass transfer of thethin recording layer caused by overlapped marks during overwriting andimproving the cycle properties of overwrite.

The total thickness of the first dielectric layer, the thin recordinglayer, the second dielectric layer and the reflecting layer is 250-430nm. The 3-15 μm thick overcoat resin layer generating tensile stress isformed on the reflecting layer. As a result, tilt of the opticalrecording medium is almost eliminated. Tilt is an angle where thehorizontal reference plane of the medium and a tangent of the surface ofthe medium formed with the layers intersect each other. In addition, byusing a transparent substrate having a convex surface, the tilt of theoptical recording medium is further minimized.

In addition, information is recorded in the MCAV recording method byvarying the record starting point at the maximum of 7.7 μm within therange of 5-12 m/s linear velocity, so that the mass transfer at the thinrecording layer becomes even and the cycle properties of overwriteimprove.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of an optical recording medium of anembodiment of the invention.

FIG. 2 (a) is a graph showing the correlation of the thickness of a thinrecording layer, amplitude and recording power.

FIG. 2 (b) is a graph showing the correlation between the thickness of asecond dielectric layer and amplitude.

FIG. 2 (c) is a graph showing the correlation between the thickness of areflecting layer and the recording power.

FIG. 3 is a graph showing the tilt of an optical recording medium of oneembodiment of the invention before and after an accelerated test.

FIG. 4 shows a waveform picture of an optical recording medium of oneembodiment of the invention after overwriting 100,000 times.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be explained by referring to figures. As shown in FIG.1, an optical recording medium of the invention consists of atransparent disk substrate 1, a first dielectric layer 2, a thinrecording layer 3, a second dielectric layer 4, a reflecting layer 5 andan overcoat protective layer 6. The second dielectric layer is thinnerthan the first dielectric layer, and the second dielectric layer and thereflecting layer are 35-70 nm thick and 70-120 nm thick respectively, sothat recording sensitivity and overwrite properties improve. The thinrecording layer is 18-30 nm, and is also so thin that heat capacity canbe minimized and information can be recorded with small power. Disksubstrate 1 shown in FIG. 1 is a transparent resin substrate, such as apolycarbonate resin substrate, or a glass substrate. First dielectriclayer 2 formed on disk substrate 1 is made of, for example, a heatresisting mixed material of ZnS and SiO₂, and is 140-210 nm thick. Thinrecording layer 3 formed on first dielectric layer 2 is made of a mixedmaterial of. GeTe, Sb₂ Te₃ and Sb, and nitrogen. The layer is 18-30 nmthick. The thin recording layer is composed so that 0.2≦b≦0.5 where b isthe mol ratio of Sb/Sb₂ Te₃. Sb has correlations with crystallizationspeed. It is not preferable to have b greater than 0.5 since the eraserate declines due to the decrease in crystallization speed. When b isless than 0.2, there is a problem in that the recording amplitude of aninner circumference having a low linear velocity is reduced. Seconddielectric layer 4 formed on thin recording layer 3 is made of the samematerial used for first dielectric layer 2, and is 35-70 nm thick.Reflecting layer 5 formed on second dielectric layer 4 utilizes themultiple interference of a laser beam, and is mainly made of Al. Thethickness of the reflecting layer is 70-120 nm. When only Al is used forthe reflecting layer, crystal grain size in the layer grows under a hightemperature and humidity environment, so that the quality of the layerdeteriorates due to intergranular corrosion. Thus, besides Al, at leastone element selected from the group comprising Ti, Ni, Cr, Cu, Ag, Au,Pt, Mg, Si snd Mo is added to the layer so as to prevent crystal growthand deterioration under the above environment. Overcoat resin protectivelayer 6 formed on reflective layer 5 cures and shrinks, thus generatingtensile stress. The layer is made of ultraviolet ray curing resin, andis formed by a spin coat method at a thickness of 3-15 μm. A vacuumevaporation method or a sputtering method is applied to form firstdielectric layer 2, second dielectric layer 4, thin recording layer Sand reflecting layer 5.

When a 35 mW semiconductor laser is used so as to record on a phasechange optical disk, the maximum output of the disk is 14 mW with 40%transmission efficiency of optical pickup. In consideration of theunevenness of optical pickup, it is necessary for a disk to record at 12mW or less. In order to improve reloading properties by increasing therecording sensitivity of a disk, not only optical properties but alsothermal properties and mechanical strength have to be considered.

The reasons for specifying the range of layer thickness are explainedbelow.

(1) The thickness of the first dielectric layer 2 will now be explained.The thicknesses of thin recording layer 3, second dielectric layer 4 andreflecting layer 5 were fixed, and the thickness of first dielectriclayer 2 was varied. Within the range of 140-210 nm thickness, theabsorption Ad of crystal with optical characteristic, the absorption Awof an amorphous state, and the difference in reflectance (ΔR) of crystalshowing a signal size and the amorphous state almost stayed the same.Recording power (C/N ratio>50 dB power) was also the same. When thethickness was outside the range, there was a problem in that ΔR andsignal amplitude became small.

(2) The thickness of the thin recording layer 3 will now be explained.FIG. 2 (a) is a graph showing recording power when the thicknesses ofthe first dielectric layer 2, the second dielectric layer 4 and thereflecting layer 5 are fixed at 170 nm, 40 nm and 70 nm respectively andthe thickness of the thin recording layer is varied. According to thegraph, when the recording power is 12 mW or less and amplitude is within±2 db, the thickness of thin recording layer 3 is 18-30 nm. When thethickness is more than 30 nm, heat capacity becomes large andsensitivity lowers. The amplitude becomes small when the thickness isless than 18 nm, thus providing an undesirable result.

(3) The thickness of the second dielectric layer 4 will now beexplained. The thickness of the layer has to be determined inconsideration of optical properties and thermal cooling speed. FIG. 2(b) is a graph showing the correlation between recording power andsignal amplitude when the thicknesses of the first dielectric layer 2,the thin recording layer 3 and the reflecting layer are fixed at 170 nm,23 nm and 70 nm respectively and the thickness of the second dielectriclayer 4 is varied. The signal amplitudes from using 70 nm or lessthickness of second dielectric layer 4 are within -3 dB of the amplitudewith 30 nm thickness. When the thickness is more than 70 nm, the signalamplitudes are undesirably small. The recording power is 12 mW or lesswhen the thickness is 30 nm or more. Recording thin layer 3 becomesclose to reflecting layer 5 when the thickness of the second dielectriclayer is less than 30 nm. Thus, more heat is likely to disperse andsensitivity declines, thus providing undesirable results.

(4) The thickness of the reflecting layer 5 will now be explained. Thethickness of the layer has to be decided in consideration of heatcapacity and mechanical strength. FIG. 2 (c) is a graph showingrecording power when the thicknesses of first dielectric layer 2, thinrecording layer 3 and second dielectric layer 4 are fixed at 170 nm, 23nm and 40 nm respectively, and the thickness of the reflecting layer isvaried. When the reflecting layer is 120 nm or less thick, recordingsensitivity with 12 mW or less recording power is obtained. Recordingpower is 9 mW and constant when the thickness is 70 nm or less. Theexperiment of overwrite cycle properties was directed to disks ofvarious reflecting layer thickness. In the experiment, random signals,modulated to "2-7" modulation by PPM (Pit Position Modulation)recording, were used. At 5 m/s linear velocity, 4.03 MHz was employed asthe maximum recording frequency, and 8.87 MHz was used as the maximumrecording frequency at 12 m/s linear velocity. The wavelength of thesemiconductor laser was 780 nm, and NA (Numerical Aperture) was 0.5.Jitter, gaps between signals to be recorded and recorded signals, wasmeasured by a time interval analyzer. As a result, when the thicknesswas less than 80 nm, it was found that the mechanical strength of thereflecting layer was weak. With 80 nm or more thickness, the layer had100,000 or more overwrites. Therefore, the thickness of the reflectinglayer is preferably 70-100 nm.

If the mixed ratio of SiO₂ in ZnS--SiO₂ constituting first and seconddielectric layers 2 and 4 is 5 mol % or less, the effect of minimizingcrystal grain size is reduced. When the ratio is 40 mol % or more, thestrength of SiO₂ becomes insufficient. Thus, it is preferable that themole fraction of SiO₂ is 5-40 mol %.

Moreover, the temperature at the inner and the outer circumferencesduring recording is the same since the recording pulse width of theinner circumference is set larger than that of the outer circumference.Thus, the sizes of recording marks become almost the same at the innerand the outer circumferences, thus minimizing mass transfer at the thinrecording layer, caused by overlapping masks, and signal deterioration.It is preferable that the pulse width of the inner circumference is40-60 ns and that of the outer circumference, 30-40 ns. Within theseranges of pulse width and within the range of 5-12 m/s for the inner andthe outer circumferences, an overwrite experiment was carried out. Thedeterioration of jitter at 100,000 or more overwrites was not found.Thus, the ratio of pulse width of the inner circumference relative tothat of the outer circumference is 1.2 or more.

In the invention, first and second dielectric layer 2 and 4 made of ZnSand SiO₂, thin recording layer 3 comprising Te, Ge, Sb and nitrogen andreflecting layer 5 made mainly of Al are formed on a 1.2 mm thicksubstrate having a 120 mm disk diameter. The total thickness of thesefour layers is 430 nm or less. When the layers are formed on a signalingsurface of the disk substrate, the surface protrudes, thus generatingcompressive stress and about 3 mrad tilt. Therefore, by forming 3-15 μmthick overcoat resin protective layer 6 generating tensile stress on thereflective layer, the compressire stress is offset. As a result, anoptical disk having little tilt is provided with only about 1-2 mradeven on the outer circumference. Overcoat resin protective layer 6cannot offset the compressire stress if its thickness is less than 3 μmbecause the strength of the layer is not sufficient. On the other hand,when the layer is more than 15 μm thick, its tensile stress is so largethat the surface of the optical disk where the layers are formed becomesconcave and the tilt of the disk becomes large. Thus, the thickness ofovercoat resin protective layer 6 is preferably 3-15 μm. A mixedmaterial of acrylic ultraviolet ray curing resin, such as urethaneacrylate, and acrylic ester monomer is used for overcoat resinprotective layer 6. However, the material for the layer is not limitedto this alone. The above-mentioned properties can be provided as long asthe material for the layer has about 10 percentage of contraction bycuring.

A substrate is molded beforehand to have a convex surface with 1-2 mradtilt (surface to be recorded with information) relative to an innercircumference reference plane. Then, first dielectric layer 2, thinrecording layer 3, second dielectric layer 4 and reflecting layer 5 areformed on the surface. The total thickness of the four layers is 430 nmor less, and the compressive stress of the layers is 0.5×10⁹ dyn/cm² orless. Ultraviolet ray curing resin is formed on reflecting layer 5 at3-15 μm thickness as overcoat protective layer 6 for generating tensilestress, thus decreasing tilt. After forming the overcoat protectivelayer on the four layers, the substrate and the layers are annealed at100° C. for one hour, so that the stresses of the substrate, the thinlayers, and the overcoat protective layer are relaxed during the formingstep. Moreover, the substrate and the layers will not be deformed. As aresult, the negative effect on recording properties caused by tilt suchas off-tracking can be minimized. The above-noted substrate, which has aconcave surface with 1-2 mrad (surface to be recorded with information)relative to an inner circumference reference plane, can be easilyprepared by means of changing the temperature distribution of a metallicmold by an injection method. In the above-mentioned method, the tilt ofa disk can be set to less than 5 mrad. The disk prepared in the methoddescribed above was placed in an environment of 80% humidity and 90° C.for 20 hours (accelerated test), and then was removed. The disk was leftto sit at room temperature, and its tilt was measured. The surface ofthe disk formed with layers warped significantly into a convex shape forseveral hours after removal. However, after 48 hours, the tilt becameless than 5 mrad even though the tilt of the outer circumference wasslightly greater than the tilt before the accelerated test (FIG. 3). Thetilt was measured by an optical disk tester: LM-110 (manufactured by OnoSokki Co., LTD, Japan).

At the 5-12 m/s linear velocity of the MCAV recording method, whichrecords by changing recording frequencies in accordance with linearvelocity, information is recorded within a range where the informationis stored in a sector by changing a record starting point, and isoverwritten evenly in the sector. Thus, mass transfer that is generatedby recording new marks over previously recorded marks becomes even. As aresult, the optical recording medium of the invention prevents thephenomenon whereby recording layers shift to a section and accumulatewhere the same signals are always recorded such as a resync section,lowering recording sensitivity. This effect of the invention issignificant at a low power side where the overlap of marks is small, sothat the power margins of overwrite properties increase. When the recordstarting point was changed at the maximum of 7.75 μm and was shifted bya 0.484 μm interval, no deterioration was found after overwriting100,000 times, as shown in FIG. 4, and the effect was significant.

The invention is explained in further detail in the following example.

EXAMPLE 1

On one surface of a 1.2 mm thick polycarbonate substrate 120 mm indiameter, a 170 nm thick dielectric layer was formed of a mixed materialof ZnS and SiO₂ that contained SiO₂ at 20 mol %. A 26 nm thick recordinglayer made of a material of Te₅₃.0 Ge₂₂.6 Sb₂₄.4 (Te:53.0 atom %,Ge:22.6 atom % and Sb:24.4 atom %)) mixed with nitrogen was formed onthe first dielectric layer. On the recording layer, a 44 nm thick seconddielectric layer was formed of the same material as the material usedfor the first dielectric layer. A 95 nm thick reflecting layer made ofAl alloy was formed on the second dielectric layer by a sputteringmethod. The high frequency sputtering method was applied to form thefirst and the second dielectric layers with 2 mTorr sputtering pressureand by using Ar gas (30 SCCM). The direct current sputtering method wasapplied to form the recording layer with 1 mTorr sputtering pressure andby using a mixed gas of Ar (15SCCM) and N₂ (0.8SCCM). The reflectinglayer was formed by the direct current sputtering method with 2 mTorrsputtering pressure and Ar gas (15SCCM). In order to protect the fourlayers, acrylic ultraviolet curing resin, such as SD101 (manufactured byDAINIPPON INK & CHEMICALS, Japan) which is a mixed material of urethaneacrylate and acrylic ester monomer, was coated on the reflecting layerto 5 μm thickness by a spin coat wave, thus preparing a single platestructure disk. The tilt of the disk was measured by an optical disktester (LM110 manufactured by Oho Sokki Co., LTD, Japan), and was 3mrad. The recording and erasing properties of the disk were measured byan optical disk drive having 2026 rpm, a 780 nm semiconductor laserwavelength and 0.5 NA. At the outermost circumference with 12 m/s linearvelocity, signals at 8.87 MHz recording frequency were recorded with 32ns pulse width. The C/N ratio was measured by a spectrum analyzer andwas 50 dB or more. The build-up power of the C/N ratio was 12 mW. 8.32MHz signals were overwritten after 8.87 MHz signals were recorded, sothat the erase ratio was obtained by subtracting spectrum in the processof overwriting 3.32 MHz signals from spectrum in the process ofrecording 8.87 MHz signals; this was 25 dB.

The cycle properties of overwrite were tested. Random signals modulatedto "2-7" by PPm recording were used to test overwrite.

The method of recording by changing a record starting point within arange of keeping recording information in a sector was applied. Therecord starting point was changed by 7.75 μm at maximum, and wasrandomly shifted at an interval of 0.484 μm within a range.

At the inner circumference with 5 m/s linear velocity, 4 MHz was appliedas the highest recording frequency. 8.87 MHz was used as the highestfrequency at the outer circumference with 12 m/s linear velocity. Jitterwas measured by a time interval analyzer. According to the measurement,no deterioration was found from the beginning to 100,000 or moreoverwrites at 5 m/s and 12 m/s linear velocity.

EXAMPLE 2

The following table shows the change in recording sensitivity andoverwrite properties of a disk when the thicknesses of the first and thesecond dielectric layers and of the reflecting layer are changed. Thelayers were formed by the method applied in Example 1.

    ______________________________________                                        Condition            Results                                                  Sample  *        **     ***    Recording                                      No.     (nm)     (nm)   (nm)   sensitivity                                                                          Overwrite                               ______________________________________                                        1       230      30     95     X      Δ                                 2       210      35     95     Δ                                                                              ◯                           3       170      43     95     ◯                                                                        ◯                           4       140      50     95     ◯                                                                        ◯                           5       130      55     95     ◯                                                                        X                                       6       170      62     95     ◯                                                                        ◯                           7       150      70     95     Δ                                                                              ◯                           8       160      75     95     Δ                                                                              X                                       9       170      43     60     ◯                                                                        X                                       10      140      53     70     ◯                                                                        ◯                           11      190      53     80     ◯                                                                        ◯                           12      150      53     90     ◯                                                                        ◯                           13      130      53     100    ◯                                                                        ◯                           14      210      53     110    ◯                                                                        ◯                           15      140      53     120    ◯                                                                        ◯                           16      170      53     130    X      ◯                           17       48      53     95     X      X                                       18       40      53     95     X      X                                       19       35      53     95     X      X                                       ______________________________________                                         *Thickness of first dielectric layer                                          **Thickness of second dielectric layer                                        **Thickness of reflecting layer                                               ◯: Good, Δ: Fair, X: Poor                              

As has been shown, the optical recording medium has high recordingsensitivity and excellent repeated recording and deleting properties.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof. The embodiments disclosedin this application are to be considered in all respects as illustrativeand not restrictive, the scope of the invention is indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

What is claimed is:
 1. An optical recording medium comprising atransparent substrate, a first dielectric layer comprising a mixedmaterial of ZnS and SiO₂ formed on one surface of said transparentsubstrate, a recording layer formed on said first dielectric layer, asecond dielectric layer comprising a mixed material of ZnS and SiO₂formed on said recording layer, and a reflecting layer formed on saidsecond dielectric layer; wherein:(i) said recording layer has propertiesof becoming amorphous after its temperature is increased, then meltingand quenching by absorbing energy from the irradiation of a laser beam,and of crystallizing its amorphous state by temperature rise; (ii) saidsecond dielectric layer is thinner than said first dielectric layer andis 35-70 nm thick; (iii) said reflecting layer is 70-120 nm thick andcomprises Al and at least one metal selected from the group consistingof Ti, Ni, Cr, Cu, Ag, Au, Pt, Mg, Si and Mo; (iv) the combinedthickness of the first dielectric layer, the recording layer, the seconddielectric layer and the reflecting layer is 250-430 nm; and (v) thecompressire stress of said first dielectric layer, said recording layer,said second dielectric layer, and said reflecting layer is 0.5×10⁹dyn/cm² or less.
 2. The optical recording medium as in claim 1, whereinthe recording layer is 18-30 nm thick.
 3. The optical recording mediumas in claim 1, wherein the first dielectric layer is 140-210 nm thick.4. The optical recording medium as in claim 1, wherein each of saidmixed material comprises SiO₂ 5-40 mol % and ZnS at 60-95 mol %.
 5. Theoptical recording medium as in claim 1, wherein the recording layercomprises Te, Ge and Sb.
 6. The optical recording medium as in claim 5,wherein the recording layer further comprises nitrogen.
 7. The opticalrecording medium as in claim 1, wherein the recording layer comprisesGeTe, Sb₂ Te₃, Sb and nitrogen.
 8. The optical recording medium as inclaim 7, wherein the recording layer is composed so that 0.2≦b≦0.5 whereb is the mol ratio of Sb/Sb₂ Te₃.
 9. The optical recording medium as inclaim 1, further comprising a 3-15 μm thick overcoat resin protectivelayer on the reflecting layer; said overcoat resin protective layergenerating tensile stress.
 10. The optical recording medium as in claim9, wherein said overcoat resin is annealed for stress relaxation. 11.The optical recording medium as in claim 1, wherein said opticalrecording medium is recorded with information on one surface; andwherein an angle where a tangent of said surface and a horizontalreference plane of said optical recording medium intersect each other is1-2 mrad so as to protrude said surface.
 12. The optical recordingmedium as in claim 1, wherein the transparent substrate has a convexsurface; wherein an angle where a tangent of said convex surface and ahorizontal reference plane of said transparent substrate intersect eachother is 1-2 mrad; and wherein the first dielectric layer, the recordinglayer, the second dielectric layer and the reflecting layer arecumulatively formed on said convex surface.